1. Field of the Invention
The present invention relates to an optical semiconductor apparatus for use in the fields of optical communication systems and so forth, such as oscillation polarization-mode switchable semiconductor lasers which can be driven by a direct modulation method with reduced dynamic wavelength fluctuation even during high-speed modulation operation, and polarization-mode insensitive optical amplifiers which can substantially equally amplify any polarized light component. The present invention also relates to a fabrication method thereof, a modulation method for modulating or driving the optical semiconductor apparatus, a light source apparatus and an optical communication system or method which use, for example, the optical semiconductor apparatus as a transmitter.
2. Related Background Art
As an oscillation polarization-mode selective or switchable dynamic single mode semiconductor laser, the following device has been developed and proposed (see, for example, Japanese Patent Laid-Open No. 5-310592 (1993)). The oscillation polarization mode of the mode switchable laser device can be modulated by a digital signal which is produced by superposing a minute-amplitude digital signal on a bias injection current. The device is a distributed feedback (DFB) laser in which a distributed reflector, made of a grating, is introduced into a semiconductor laser resonator or cavity, and its property of wavelength selectivity is utilized therein. In the device, strain is introduced into an active layer of a quantum well structure, or the Bragg wavelength is located at a position lower than the peak wavelength of its gain spectrum, so that gains for the transverse electric (TE) mode and the transverse magnetic (TM) mode are approximately equal to each other for light at wavelengths close to an oscillation wavelength, under a current injection condition near its oscillation threshold. Further, a plurality of electrodes are arranged and currents are unevenly injected through those electrodes. An equivalent refractive index of the cavity is unevenly distributed by the uneven current injection, and oscillation occurs in one of the TE mode and the TM mode and at a wavelength which satisfies a phase matching condition and enjoys a minimum threshold gain. When the balance of the uneven current injection is slightly changed to vary the competing relationship of the phase condition between the TE mode and the TM mode, the oscillation polarization mode and the wavelength of the device can be switched.
In that semiconductor device, an antireflection coating is provided on one end facet to asymmetrically employ effects of the uneven current injection between its light-output side and its modulation-current supply side. Alternatively, lengths of the electrodes are made different to introduce effectively structural asymmetry.
Furthermore, Japanese Patent Laid-Open No. 2-117190 discloses a semiconductor laser apparatus in which two semiconductor devices are arranged serially or in parallel. One of them principally oscillates or amplifies a wave in a predetermined polarization mode, and the other one chiefly oscillates or amplifies a wave in another polarization mode. Those devices are provided in a common layer or in parallel layers.
However, the above-discussed conventional oscillation polarization-mode switchable DFB semiconductor laser, which selects the oscillation polarization mode depending on the phase condition, is sensitive to the phase at the end facet. As a result, (1) the oscillation wavelength and polarization mode of the device depend on the current injection condition in a complicated way, and (2) variance in characteristics concerning the oscillation polarization mode and so forth appears among the individual devices. If antireflection coatings were to be provided on both end facets to solve those disadvantages (1) and (2), asymmetry in a light propagation direction of the device would be weakened and the effect of the uneven current injection would be reduced. Thus, stable switching of the oscillation polarization mode would be lost.
In the apparatus of Japanese Patent Laid-Open No. 2-117190, the oscillation or amplification of the wave in a predetermined polarization mode is performed by selecting a device's geometric shape. As a result, its yield is decreased, depending on the variance in etched depth and ridge width that occurs during the ridge fabrication process.